1. Field of the Invention
The present invention relates to a transceiver, more particularly to carrier leakage compensation in a transmit branch of a time division duplex transceiver that has a common local oscillator for the transmit branch and the receive branch of the transceiver. Such a transceiver can be a transceiver operating in the so-called 2.4 GHz ISM band, or any other suitable transceiver.
2. Description of the Related Art
In the U.S. Pat. No. 4,717,894 calibration of a vector modulator is disclosed using a scalar detector to measure the amplitude of an RF output signal of a transmitter. Calibration is done to compensate errors in vector modulated signals such as errors caused by carrier leakage, amplitude imbalance of the I and Q channels, and a quadrature phase error. Carrier leakage is the appearance of an output signal from the quadrature mixers even when the modulation inputs are grounded and can be caused by a non-zero offset voltage in a mixer, capacitive or inductive coupling of the LO signal across the mixer, and impedance mismatches at the mixer ports. Calibration measurements are made using a detector to measure the amplitude of the RF output signal. DC signal sources provide reference signals for the I and Q modulation inputs, when connected by switches to mixers and in place of the I and Q modulation signals. Other DC sources provide carrier leakage compensation signals for the I and Q signal components, respectively. A signal source, which can be selectively connected to either the I or Q input lines by switches provides calibration signals for balancing the I and Q amplitude. Both phase and amplitude errors are corrected, and carrier leakage is minimized. The carrier leakage is minimized by adjusting the DC sources for carrier leakage compensation to minimize the RF output when the modulation inputs are grounded. For calibrating the vector modulator thus a power detector is needed to measure the RF power of the transmitter. Such a power detector renders the vector modulator more complicated and expensive.
In time division duplexing transceivers, in order to reduce cost, it is desirable to have a common local oscillator for the transmitter and the receiver. Furthermore, in order to achieve a short transmit-receive turn-around time, the same synthesizer based local oscillator frequency should be used for the transmitter and the receiver. Moreover, in order to be able to integrate receive channel filters, or to produce transmitter base band I and Q signals via digital-to-analog converters while applying a low sampling rate, the intermediate frequency should be kept low with respect to the radio frequency. Particularly when such a common-LO and low-IF transceiver operates at a microwave frequency range, e.g. at a 2.4 GHz ISM band, an uncalibrated transceiver has considerable carrier leakage in the transmitter. With a zero-IF or low-IF frequency, the carrier leakage frequency falls within the transmission band and thus cannot be removed by the transmitter of RF antenna filter. Another problem related to carrier leakage is that the transmitter may fail modulation spectral emission requirements as imposed on transceivers by a standard, e.g., the FCC requirements for transceivers operating in the 2.4 GHz ISM band. Particularly, when the carrier leakage falls in adjacent channels of such an ISM band, due to a largely reduced spectral emission headroom by the modulation spectrum, the latter problem may be quite significant.
It is an object of the invention to provide a transceiver not needing an RF power detector that measures the output power of a transmitter, to calibrate the transmitter part of the transceiver.
It is another object to calibrate a transmitter in a transceiver such that the transceiver fulfills emission requirements as defined in standards, such as the 2.4 GHz standard for the ISM band.
It is another object of the invention to provide a transceiver virtually only using components of a standard transceiver to calibrate the transmitter part of the transceiver.
It is still another object of the invention, during calibration of the transmitter, after DC-nulling of the receiver, to calibrate the transmitter on the basis of measured DC-errors induced in the receiver by the transmitter, relative to a DC-nulling reference determined during DC-nulling.
It is still another object of the invention to provide a transceiver wherein, in addition to carrier leakage compensation, also imbalances in the transmitter digital-to-analog converters, that provide transmit signals, are compensated.
In accordance with the invention, a transceiver is provided comprising:
a transmitter; a receiver, said receiver comprising a DC-nuller providing a nulling-output;
a common local oscillator coupled to said transmitter and said received;
a storage; and
a controller that is coupled to said transmitter and to said receiver,
said controller being configured to control said DC-nuller, with said transmitter switched off and said receiver switched on, to obtain as a DC-nulling reference a value of said nulling-output that is equal to a setpoint, to store into said storage said DC-nulling reference, and to freeze said nulling-output to said setpoint,
said controller further being configured to inject a DC-signal into said transmitter, with said transmitter switched on, to modify said injected DC-signal until said frozen nulling output is equal to said stored DC-nulling reference, and to store said injected DC-signal into said storage.
Preferably, a feedback or servo loop is used in AC-couplers of the receiver to perform the DC-nulling, while the transmitter is switched off. After DC-nulling of the receiver, and storage of the determined reference, the transmitter is switched on to perform calibration via the receiver. The servo has an analog storage to freeze the DC-nulling state determined with the transmitter switched off.
Preferably, during injection of the DC-signals in the I and Q paths of the transmitter, between first mixers output a low IF signal and second, RF mixers, for instance, a single tone modulation signal is inputted to the modulator inputs. Then, an average value of the nulling-output is measured. Herewith, advantageously, in addition to carrier leakage compensation, also imbalances in the transmitter digital-to-analog converters, that provide transmit signals, are compensated.
Preferably, the receiver is a quadrature receiver and the transmitter is a quadrature transmitter.
In order to avoid that the calibration is disturbed by signals picked up by an antenna, a receiver in a transceiver with an antenna is isolated from the antenna.
The calibration of the invention can advantageously be done in zero-IF and low-IF transceivers, a zero-IF carrier leakage manifesting itself in the middle of the desired transmit band, and at low-IF carrier leakage still being in the desired transmit band.